Concrete walls, and other concrete structures and objects, traditionally are made by building a form or a mold. The forms and molds are usually made from wood, plywood, metal and other structural members. Unhardened (plastic) concrete is poured into the space defined by opposed spaced form members. Once the concrete hardens sufficiently, although not completely, the forms are removed leaving a concrete wall, or other concrete structure, structural member or concrete object, exposed to ambient temperatures. The unprotected concrete walls, structures or objects are then exposed to the elements during the remainder of the curing process. The exposure of the concrete to the elements, especially temperature variations, often makes the curing of the concrete a slow process and the ultimate strength difficult to control or predict. To compensate for these losses, larger amounts of portland cement sometimes are used than otherwise would be necessary in order to insure sufficient concrete strength is achieved.
The curing of plastic concrete requires two elements, water and heat, to fully hydrate the cementitious material. The curing of plastic concrete is an exothermic process. This heat is produced by the hydration of the portland cement, or other cementitious materials, that make up the concrete. Initially, the hydration process produces a relatively large amount of heat. As the hydration process proceeds, the rate of hydration slows thereby reducing the rate of heat production. At the same time, moisture in the concrete is lost to the environment. If one monitors the temperature of concrete during the curing process, it produces a relatively large increase in temperature which then decreases rapidly over time. This chemical reaction is temperature dependent. That is, the hydration process, and consequently the strength gain, proceeds faster at higher temperature and slower at lower temperature. In traditional curing of concrete, first, the heat is lost which slows the hydration process; then, the moisture is lost making it difficult for the cementitious material to fully hydrate, and, therefore, impossible for the concrete to achieve its maxim strength.
Concrete in conventional concrete forms or molds is typically exposed to the elements. Conventional forms or molds provide little insulation to the concrete contained therein. Therefore, heat produced within the concrete form or mold due to the hydration process usually is lost through a conventional concrete form or mold relatively quickly. Thus, the temperature of the plastic concrete may initially rise 20 to 40° C., or more, above ambient temperature due to the initial hydration process and then fall relatively quickly to ambient temperature, such as within 12 to 36 hours. This initial relatively large temperature drop may result is concrete shrinkage and/or concrete cracking. The remainder of the curing process then proceeds at approximately ambient temperatures, because the relatively small amount of additional heat produced by the remaining hydration process is relatively quickly lost through the uninsulated concrete form or mold. The concrete is therefore subjected to the hourly or daily fluctuations of ambient temperature from hour-to-hour, from day-to-night and from day-to-day. Failure to cure the concrete under ideal temperature and moisture conditions affects the ultimate strength and durability of the concrete. In colder weather, concrete work may even come to a halt since concrete will freeze, or not gain much strength at all, at relatively low temperatures. By definition (ACI 306), cold weather conditions exist when “ . . . for more than 3 consecutive days, the average daily temperature is less than 40 degrees Fahrenheit and the air temperature is not greater than 50 degrees Fahrenheit for more than one-half of any 24 hour period.” Therefore, in order for hydration to take place, the temperature of concrete must be above 40° F.; below 40° F., the hydration process slows and at some point may stop altogether. It is typically recommended that concrete by moisture cured for 28 days to fully hydrate the concrete. However, this is seldom possible to achieve in commercial practice.
Insulated concrete form systems are known in the prior art and typically are made from a plurality of modular form members. U.S. Pat. Nos. 5,497,592; 5,809,725; 6,668,503; 6,898,912 and 7,124,547 (the disclosures of which are all incorporated herein by reference) are exemplary of prior art modular insulated concrete form systems. Full-height insulated concrete forms are also known in the prior art. U.S. Pat. No. 8,555,583 (the disclosure of which is incorporated herein by reference) discloses a full-height insulated concrete form.
Insulated concrete forms or molds reduce the heat transmission to and from the concrete within such forms or molds. However, some heat may still manage to escape or penetrate the insulation and thereby affect the temperature of the concrete therein. Concrete will not cure to its maximum strength and durability unless it is cured under proper temperature conditions.
Electrically heated insulating blankets are known in the prior art, such as those disclosed in U.S. Pat. Nos. 7,183,524 and 7,230,213. Such electrically heated insulating blankets are known for use in northern climates for thawing frozen ground and preventing curing concrete from freezing. It is know that plastic concrete will not cure satisfactorily at temperature below 50° F. However, such electrically heated blankets are designed to provide a constant amount of heat to the plastic concrete and are used only for the purpose of preventing the concrete from freezing in cold weather.
U.S. Pat. No. 5,707,179 discloses a system using water in pipes placed within plastic concrete for either heating or cooling the concrete. Again, the objective of this system is to maintain the concrete at a sufficiently high temperature so that it will cure when the ambient conditions would otherwise prevent proper curing. This system however is not practical. Furthermore, this system does not disclose adjusting the amount of heat provided to the concrete as a function of time so that the concrete temperature follows a predetermined temperature profile.
Another problem exists when large volumes of concrete are placed in forms or molds. In such cases, the interior portion of the concrete may heat more quickly and cool more slowly than the outer portion of the concrete. The difference in the rate of heating/cooling between the interior and outer portions produces a temperature differential between the interior portion of the concrete and the outer portion of the concrete. If the temperature differential exceeds a certain amount, cracking of the concrete may result. This problem is frequently found in mass concrete. The ACI Committee defines “mass concrete” as “any large volume of cast-in-place concrete with dimensions large enough to require that measures be taken to cope with the generation of heat and attendant volume change to minimize cracking” Previous attempts at controlling this problem include refining concrete mix proportions, using aggregate with desirable thermal properties, pre-cooling the concrete constituent materials, cooling the plastic concrete with liquid nitrogen, using internal water-filled pipes to cool the concrete itself, and placing the concrete in several lifts or pours. These approaches are not entirely desirable or successful since they do not control the temperature differential between the surface and the core of the concrete mass. They can also be expensive to implement in practice.
Therefore, it would be desirable to produce a concrete forming or molding system that controls the temperature of curing concrete at predetermined levels over time. It would also be desirable to provide a concrete curing system that adjusts the temperature of curing concrete in a forming or molding system so that the temperature follows a predetermined temperature profile over time. It would also be desirable to provide a concrete curing system that accelerates concrete maturity or equivalent age to achieve improved concrete strength, particularly early concrete strength. It would also be desirable to provide a concrete curing system that adjusts the temperature differential of concrete in a forming or molding system, especially mass concrete.